Iron removal from process solutions may be considered to be the technical and economic crux of hydrometallurgical processing. This is because large volumes of waste can be generated through precipitation steps, at the further expense of high reagent costs, water consumption, and final waste impoundment. While tenable for heap leach operations (due to the small quantities of iron in such heap leach operations), the amount of iron that can be generated during chalcopyrite leaching, zinc leaching, and/or nickel laterite leaching can make it cost prohibitive to treat/remove soluble iron from a hydromet circuit using traditional methods. In particular, iron(II) (i.e., Fe2+) can substantially build up in chalcopyrite leach circuits and heap leach circuits, since raffinate-recycle streams coming from solvent extraction/electrowinning (SX/EW) circuits aren't efficiently configured to handle or remove excess residual iron economically.
The predominate oxidation state of iron during chalcopyrite leaching and heap leaching is +2. Accordingly, a costly oxidation step would generally be required prior to the precipitation of iron in such leaching circuits. For nickel laterite leaching, buildup of iron(III) in solution may require very expensive, high temperature autoclave operations, and these high temperatures tend to drive reactions towards the formation of less desirable species such as basic iron sulfate and hematite.
Accordingly, in order to make certain leaching methods more economical and/or able to produce more desirable iron-based byproducts (e.g., magnetite), an alternative method for iron removal is proposed herein. This novel alternative method of removing iron may apply to chalcopyrite leaching as well as other hydrometallurgical applications where iron removal is essential (e.g., acid mine drainage, zinc leaching, nickel laterite leaching, etc.), without limitation.
Current state-of-the-art iron removal practices generally involve only a two-step pH change; wherein, in a first stage, limestone or lime may be used to neutralize free acid and precipitate gypsum and iron as an iron hydroxide, preferably goethite. Other metals, such as zinc (Zn), Magnesium (Mg), and Manganese (Mn), etc., may then be precipitated in a second stage having a pH greater than 9. With these conventional two-stage impurity removal processes, low-density/high-surface area ferric hydroxides are formed, which are typically difficult to settle and filter. Moreover, with these conventional two-stage iron removal processes, metal values still in solution along with the precipitates are typically disposed of as tails and are rarely recovered. Moreover, with these conventional two-stage iron removal processes, all copper present in post-iron precipitation treated liquors can be lost to tailings, and therefore, streams that are selected for iron removal, or that can be adequately treated for iron removal are limited (this is especially true for copper-leaching circuits). The voluminous nature of the ferric hydroxide precipitates, high water entrainment, and significant copper losses make iron removal a costly endeavor from both an economic and environmental perspective. Moreover, with conventional iron-removal processing, large tailings impoundments are typically necessary and with the added disadvantage of significant water inventory trapped as hydrated iron species.
In short, conventional iron removal processes promote the formation of iron hydroxides that filter poorly, upset flowsheet water balance, co-precipitate with copper, lead to metal value losses, and fail to provide desirable iron byproducts.
The following references may be relevant to this application: U.S. Pat. No. 4,150,095 A (KUNDA WASYL ET AL); WO 2007/071020 A1 (HARRIS G BRYN ET AL); CHANG Y ET AL: “Removal of iron from acidic leach liquor of lateritic nickel ore by goethite precipitate”, HYDROMETALLURGY, ELSEVIER SCIENTIFIC PUBLISHING CY. AMSTERDAM, NL, vol. 101, no. 1-2, 1 Feb. 2010 (2010-02-01), pages 84-87, XP026851826, ISSN: 0304-386X [retrieved on 2009 Nov. 27]; and, HAN HAISHENG ET AL: “Magnetite precipitation for iron removal from nickel-rich solutions in hydrometallurgy process”, HYDROMETALLURGY, ELSEVIER SCIENTIFIC PUBLISHING CY. AMSTERDAM, NL, vol. 165, 22 Jan. 2016 (2016-01-22), pages 318-322, XP029700384, ISSN: 0304-386X, DOI: 10.1016/J.HYDROMET.2016.01.006